In general, an image forming apparatus, such as a printer or a copier, and an image scanning apparatus each include a sheet feeding device that separates sheets (documents) stacked on, for example, a sheet feed cassette or a sheet feed tray on a one-by-one basis and feeds the separated sheet to an image forming portion or a document scanning portion.
Such a sheet feeding device includes a separating portion based on any of various methods for separation in order to separate sheets one by one. For example, air separation and frictional separation have been known. According to the former method, air is blown onto stacked sheets to separate the sheets and, after that, the sheets are conveyed one by one while being sucked. According to the latter method, sheets are separated using a sheet feed roller, a friction force applied by a frictional member, and a friction force between sheets. In addition, slope separation has also been known. According to this method, the leading edges of sheets are allowed to abut against a separation slope to separate the sheets.
A configuration proposed as a special separation method is disclosed in PTL 1. While part of the uppermost sheet of a stack of sheets is being pressed downward by a pressing member, a sheet feed roller, positioned downstream of the pressing member, abutting against the sheet feeds the sheet in the direction opposite to a sheet feeding direction. Accordingly, the uppermost sheet is moved backward while a loop is being formed in the sheet and the leading edge of the sheet is moved from the lower side of the sheet feed roller to the upper side thereof. Thus, the sheet is separated and fed.
In recent years, there have been increasing needs to install and use a small and low-cost printer or copier on a table at office or home. Related-art printers and copiers have not sufficiently met the needs. An approach to meeting the needs is to reduce the cost and size of a sheet feeding device provided for a printer or copier.
As regards a related-art air separation sheet feeding device, it requires a device for generating air pressure, thus resulting in an increase in size of the feeding device. Accordingly, it is difficult to use this feeding device in small printers and copiers. As regards related-art frictional separation sheet feeding devices, for example, a separation pad type requires an intermediate plate unit for stacking sheets and pressing the sheets against the sheet feed roller to generate sheet feed pressure, and a separation pad in addition to a sheet feed roller. A retard separation type requires a pick roller and a retard roller in addition to the intermediate plate unit and the sheet feed roller. Accordingly, such frictional separation sheet feeding devices are generally expensive, although lower than the air separation type.
The slope separation sheet feeding device is inexpensive and it is relatively easy to reduce the size of the feeding device but ease of use for a user is poor. Specifically, upon setting a bundle of sheets, the leading edges of the set sheets are allowed to abut against the separation slope. Accordingly, the sense of abutment is very small. In some cases, the sheets are set while the leading ends thereof are lying over the separation slope by accident. In this case, disadvantageously, multiple-sheet feeding (feeding of overlapping sheets) often occurs during a sheet feeding operation.
A sheet feeding device disclosed in PTL 1 requires a first sheet feed roller and a second sheet feed roller, a sensor that detects the shift of a sheet from the lower side to the upper side of the first sheet feed roller, a mechanism that swings the first sheet feed roller after detection of the shift, and a movable mechanism that presses a bundle of sheets. In addition, an upward space is needed because a loop to be formed in a sheet is large. Disadvantageously, the cost and size of such a sheet feeding device are not sufficiently reduced.